1. Field of the Invention
Illustrative embodiments of the present invention relate to a recording-medium positioning device and an image forming apparatus employing the recording-medium positioning device, and more specifically, to a recording-medium positioning device capable of preventing image-fixing failure and a reduction in service life of a fixing member due to damage caused by edge portions of a recording medium, and an image forming apparatus employing the recording-medium positioning device.
2. Description of the Background
Multi-color image forming apparatuses, such as a full-color printer and a spot-color printer, are used as copiers, printers, facsimile machines, plotters, and multi-functional devices combining several of the foregoing capabilities. A conventional color image forming apparatus includes a plurality of photoconductive drums arranged in tandem in a travel direction of an endless intermediate transfer belt. When electrostatic latent images are formed on the photoconductive drums, different color toners are adhered to the electrostatic latent images to form toner images on the photoconductive drums. The toner images on the photoconductive drums are transferred in turn onto a transfer belt.
Such a conventional image forming apparatus feeds a sheet from a sheet feed unit, conveys the sheet toward a transfer position of the toner image, temporarily stops the sheet with a pair of registration rollers located on an upstream side of the transfer position in a direction (hereinafter, a “sheet conveyance direction”) in which the sheet is conveyed, and drives the pair of registration rollers to feed the sheet to the transfer position in sync with the toner image conveyed on the transfer belt.
In such a conventional image forming apparatus, in order to align the print-start positions of a conveyed sheet and an image in a sheet width direction perpendicular to the sheet conveyance direction, a shift mechanism is provided at an upstream side of a pair of registration rollers to shift the sheet in the sheet width direction within a sheet placement area. When the print start positions of the sheet and the image are aligned, the conventional image forming apparatus feeds the sheet to a transfer position with the pair of registration rollers to transfer the image onto the sheet at the transfer position.
The sheet having the transferred image is conveyed to a fixing device and pressed and heated with a fixing member. In the fixing device, a sheet conveyance line is invariably maintained. However, after a number of sheets of identical width is fixed, certain portions of the fixing member experience wear due to repeated contact with edges of the sheets. Such worn portions may degrade the quality of an image fixed on the sheet or reduce the service life of the fixing member.
To extend the service life of the fixing member, one conventional, image forming apparatus includes a plurality of fixing devices arranged in tandem or parallel (one to another) in a sheet conveyance direction, and switches the plurality of fixing devices to perform fixing operation. However, such an arrangement involving a plurality of fixing devices is not advantageous in terms of size reduction of the image forming apparatus.
Alternatively, to suppress wear of such a fixing member by edge portions of a recording medium, another conventional image forming apparatus includes an entry-position shift unit at an upstream side of a fixing device. The entry-position shift unit shifts an entry position, at which the recording medium entries a nip portion of a fixing member, in a direction perpendicular to a sheet conveyance direction.
However, since the width of a sheet conveyance line shifted by the entry-position shift unit is a fixed value, the recording medium invariably passes several specific, fixed portions of the fixing member. Consequently, after a number of recording media passes the fixing device, these specific fixed portions of the fixing member experience wear due to repeated contact with edges of the recording media, degrading image quality at the specific worn portions.
FIGS. 11A to 11E illustrates conveyance of a sheet P using the above-described conventional entry-position shift unit. The sheet P is conveyed from a sheet-feed cassette to a conveyance belt 28 along a center line of the conveyance belt 28, which is driven by a belt drive motor 68, not shown. At this time, the sheet P is on a conveyance guide 32, and is conveyed by conveyance sub-belts 28A and 28B rotating at the same speed, as shown in FIG. 11A. As illustrated in FIG. 11B, when the rotation speed V1 of the belt drive motor 68 slows down, the rotation speed V2 of the conveyance sub-belt 28B becomes faster than the rotation speed of the conveyance sub-belt 28A. As a result, as illustrated in FIG. 11C, the, conveyance speed of the right side of the sheet P becomes faster than the conveyance speed of the left side of the sheet P, and therefore-the sheet P is tilted with respect to the conveyance direction of the sheet P and shifted toward the left side, as shown in FIG. 11C. As a result, the sheet P is conveyed with an edge portion of the sheet P abutted against a guide rib 32H as illustrated in FIG. 11D. In such a case, the sheet conveyance line varies depending on the size, type,.and thickness of sheet, causing a plurality of worn portions in end portions “Z” of the fixing member, as shown in FIG. 11E.
Such worn portions cause uneven fixing at the end portions “Z”of the fixing member, resulting in uneven image quality. FIGS. 12A and 12B illustrate such worn states of a fixing member in the related art. FIG. 12A illustrates an example of a worn portion 111 observed when one sheet passes a fixing member 110. FIG. 12B illustrates a worn state observed when a number of sheets passes the fixing member 110. In FIG. 12B, two worn portions 111 and 112 arise in the fixing member 110. When an image is fixed with the fixing member 110 in such a state, the worn portions 111 and 112 may cause uneven fixing, resulting in uneven image quality.